1. Field
The present invention relates to a laminate which exhibits strong interlaminar adhesiveness upon extrusion lamination of a thermoplastic resin onto a plastic film or a substrate including a plastic film. More particularly, the present invention relates to a laminate which exhibits strong adhesiveness by direct extrusion lamination of a thermoplastic resin onto a plastic film or a substrate without applying an anchor coating agent, an adhesive or the like to the plastic film or substrate and without ozonation of the thermoplastic resin at the time of extrusion lamination.
2. Description of the Related Art
A multilayer laminate employed in packaging, for example, is used in a laminate structure of a plastic film, a metallic foil, paper or the like depending on required physical properties. For example, a polyester film is excellent in mechanical properties, dimensional stability and toughness and is thus used as a substrate, and is laminated with a metal foil or an inorganic deposited film to impart gas barrier properties thereto and laminated with a heat-sealing resin layer to impart heat-sealing properties thereto as a packaging material.
The method of producing such a laminated film having a heat-sealing resin layer laminated on a substrate includes dry lamination, wet lamination, extrusion lamination, etc., among which the extrusion lamination advantageous in cost and efficiency is widely used.
In the extrusion lamination, a thermoplastic resin, such as polyethylene, polypropylene and an ethylene-based copolymer, is generally used. However, which such thermoplastic resin is directly laminated onto a plastic film as a substrate, they show poor adhesiveness. Thus, the plastic film is coated in advance with an anchor coating (AC) agent and then extrusion-laminated thereon with the thermoplastic resin.
However, the AC agent is applied after dilution with an organic solvent such as ethyl acetate or toluene as a diluent, and thus generally has problems such as compliance with the regulation by Fire Defense Law, deterioration in working conditions, an increase in production costs due to use of a relatively expensive coating agent, and an odor of a residual organic solvent in final products.
When the thermoplastic resin is extrusion-molded onto both sides of a plastic film as an interlayer, it may be necessary to apply an AC agent in advance on both sides of the plastic film. However, facilities capable of simultaneously applying an AC agent to both sides of the plastic film are special. On the other hand, when usual facilities for extrusion lamination of a thermoplastic resin onto one side of a substrate are used to produce a laminate of such structure, there is a problem that the number of steps should be increased.
As the method of extrusion-laminating a thermoplastic resin on both sides of a substrate without using an AC agent, many attempts have been made. As examples of such methods, there are known a technique of using an ethylene-methacrylic acid copolymer to bond it firmly to an aluminum foil by the action of its acid component and a technique of extruding, onto a deposited substrate, a modified resin having an unsaturated carboxylic acid grafted onto a polypropylene resin to improve adhesion strength. In these methods, however, resins and substrates that can be used are limited, and thus a combination of a general plastic substrate and a thermoplastic resin such as a polyethylene resin is not applicable.
Recently, a method of attaining excellent adhesiveness of a resin to a substrate without an AC agent, by ozonating the surface of its molten resin at the time of extrusion lamination is disclosed as shown in Japanese Patent Nos. 2,895,917, 3,385,663 and 3,716,006. In this method, the adhesiveness of a resin is Improved by blowing ozone to the resin, but ozone has problems such as a very bad smell and toxicity to require attention on safety, and difficulty in confirming whether ozonation is uniformly conducted.
To solve these problems, Jpn. Pat. Appln. KOKAI Publication No. 2000-238195 discloses a method which includes disposing a printed layer on a substrate and then surface-treating the printed layer with UV rays followed by extrusion lamination. However, this method always requires a printed layer and is hardly applicable to a constitution not provided with a printed layer. Even if the steps of lamination with a printed layer and of surface treatment are conducted in line, there is a problem that the steps and costs are increased in the constitution not requiring a printed layer.
A transparent gas barrier film frequently used in recent years as a packaging film has a gas barrier coating layer laminated on one side or both sides of a film. However, when the adhesiveness between the gas barrier coating layer and a resin extrusion-laminated thereon is to be exhibited, there is the same problem as in the case of the substrate described above.